WASHINGTON (CN) - The U.S. Fish and Wildlife Service plans to list 35 plant species and three tree snail species on the Hawaiian Islands of Molokai, Lanai and Maui as endangered under the Endangered Species Act.

More than half of the species have been waiting on the USFWS's candidate species list where it places species it has determined should be listed but which are precluded by higher priority listing actions.

The USFWS also plans to designate more than 270,000 acres on four islands as critical habitat for the species it proposes to list and nearly 100 other species already protected under the act.

The USFWS chose the species it would protect based on the severity of the threats faced, and which could be protected by designating critical habitat for an entire ecosystem rather than for just one species at a time.

The USFWS calls 15 of the plant species "the rarest of the rare"; they have fewer than 50 known individuals remaining in the wild. These plants are included in the mutli-jurisdictional Plant Extinction Prevention Program, which seeks to protect the species wherever examples are found.

The most common threats faced by the species are competition from invasive species and loss of habitat to nonnative ungulates like pigs, sheep, deer and cattle.

The listing proposal is in response to a May 2004 petition from the Center for Biological Diversity to list 225 species of plants and animals as threatened or endangered under the act.